Sputter coating is a coating process that involves the transport of almost any material from a source, called the target, to a substrate of almost any material. The process takes place in a reduced pressure chamber containing argon or other process gas. The reduced pressure, or vacuum, is needed to increase the distance that the sputtered atoms can travel without undergoing collision with each other or with other particles. The argon gas is ionized which results in a bluish-purple glow of a plasma. Ejection of target source material is accomplished by bombardment of the target surface with gas ions that have been accelerated toward the target by a high voltage. As a result of momentum transfer between incident ion and target, particles of atomic dimension are ejected from the target. These ejected particles traverse the vacuum chamber and are subsequently deposited on a substrate as a thin film. A similar process is generally used in sputter etching, however, the target is replaced by the object to be etched.
Radio frequency (RF) power, introduced into a process chamber via an inductive coil encircling the chamber, is often advantageously used in sputter coating and etching to enhance the development of the plasma. RF sputter coating and etching allows the deposition of insulating as well as conductive materials and the etching of substrates, utilizing lower voltages, such as 500 to 2,500 V, to accelerate the argon gas ions to the target or substrate being etched. The lower voltage at the same power provides higher deposition and etching rates with reduced substrate damage. In addition, RF power may be used to bias the substrate to change the characteristics of film deposition, especially for insulating films. Such RF power biasing of the substrate can improve adhesion and the added heat due to the bias power can provide higher mobility of source material on the substrate surface which can improve step coverage. Lower resistivity and changes in film stress can be obtained with RF voltage bias, as well. Gas incorporation into the film is usually increased. Oxide films such as silicon dioxide will have improved optical qualities and higher density when RF bias is used during deposition.
Electronically isolating certain system components with respect to RF energization is desirable when using RF power in sputtering and etching. Otherwise, the plasma may be altered, adversely affecting the sputtering or etching process. In the case of RF biasing of the substrate, lack of RF isolation can result in undesired RF power dissipation. Consequently, it is desirable to RF isolate certain components within a sputtering or etching system. This RF isolation provides an interruption of a potential RF path to ground from the RF energized component.
Another aspect of sputtering and etching systems is the requirement to thermally condition (heat or cool) certain system components or the substrate. This is usually accomplished by circulating a liquid heat transfer medium in heat transfer relationship to the system component, or substrate support, as the case may be. As previously mentioned, the substrate may be biased by RF power for improved deposition or etching characteristics. Under such conditions, the heat transfer components must be electrically isolated from the RF applied to the substrate support. For example, in sputter coating target cooling is often desirable, whereas in sputter etching wafer cooling may be needed. In such situation, a liquid coolant is circulated through the susceptor or wafer support, as the case may be. In wafer processing configurations wherein an RF coil is placed around the sputtering or etching chamber to create and/or enhance the plasma, the RF coil has resistance that generates heat, requiring the RF coil to be cooled by circulating a liquid heat transfer coolant medium through the RF coil. The heat transfer component must be isolated from the RF applied to the RF coil.
A problem arises when cooling an RF coil and/or heating or cooling a substrate support or target holder as a result of the fact that the heat transfer media, usually water, in the lines used for circulating the heat transfer media through the coil or substrate support or target interferes with RF isolation of components such as the RF coil, substrate support, target holder, etc. Maintaining the desired RF isolation while circulating a liquid heat transfer medium is required to prevent degradation of the sputtering or etching process.
Water is typically used as the heat transfer liquid medium due to the safety, low cost, and ready availability of water. For example, in a simple open cooling system, cool tap water is supplied through a supply line to the sputtering or etching system component to be cooled at which place the water is circulated and thereafter discharged to a drain via a discharge line. Tap water typically contains dissolved minerals that cause the water to be less chemically reactive to many materials. However, the dissolved minerals also create ions in the water that make the water electrically conductive, which can be disadvantageous when heating or cooling sputtering or etching system components energized with RF. Consequently, in many applications, the electrically conductive tap water is first purified to remove the minerals, making it resistive. Resistive water is corrosive to many materials such as metals. As a consequence, a balance is sought in the level of water purification to achieve an acceptable level of conductivity versus corrosiveness. In addition, chemical additives are generally added to the water to mitigate the corrosive effects, incurring additional expense and inconvenience in preparation and disposal of the liquid. These additives in some instances introduce detrimental effects such as reducing the resistivity of the water or decreasing the environmental safety of the liquid. Moreover, systems utilizing this type of component cooling or heating have to be designed to accommodate a certain amount of electrical power loss through the liquid heat transfer medium.
Achieving the requisite balance in water purity involves the expense of buying or processing the water to the appropriate purity level. Filtering and monitoring of the water is then required to maintain the purity within the acceptable range. Even with these additional requirements, there is still some degradation of performance and reliability of the components heated or cooled by the resistive water.
Other relatively abundant and environmentally safe heat transfer liquids are also available; however, many of these are also electrically conductive, but by their nature cannot be processed to a more resistive condition and are thus inappropriate for use.
Relying on the resistivity of the water to provide the RF isolation dictates that the water supply and discharge lines be increased in length, since the electrical resistance afforded by the water is a function of water path length to ground, assuming the resistivity of the water and diameter of the water lines are fixed. For example, typical resistive water cooling systems employ 12 to 13 feet of polypropylene tubing of about a quarter inch in outer diameter in each of the water supply and discharge lines to generate 1 M.OMEGA. of resistance.
Consequently, what is needed are high impedance liquid heat transfer medium supply and discharge lines that are not dependent principally upon the resistivity of the heat transfer liquid in isolating radio frequency energized components.